CHAPTER XVI 

 THE BLOOD 



The blood has several well-defined services. It is 

 a carrier of food and of waste. It receives the food from 

 the alimentary canal and bears it away to places of 

 storage or to tissues where it is to be oxidized. It re- 

 ceives waste from the active tissues and transports it 

 to the organs of excretion, especially the lungs and the 

 kidneys, through which it is eliminated. The lungs 

 have a double function since the blood in passing through 

 them not only shakes off the chief oxidized waste-product 

 of the body, carbon dioxid, but gains oxygen in its place. 



The blood is also a carrier of compounds which can 

 hardly be classified as foods or as wastes, the hormones. 

 The conception of a hormone has been suggested in con- 

 nection with the secretin formed in the epithelial cells 

 of the duodenum. A hormone passes from the place 

 of its origin to another locality and modifies the be- 

 havior of some organ or tissue. The importance of such 

 agents is more and more widely recognized. Where 

 we formerly believed that the nervous system furnished 

 the principal bonds between different parts of the body 

 we now attribute to hormones many of the influences 

 which clearly proceed from certain organs to others. 



One service of the circulating blood which is often 

 ignored is the distribution of heat. The active tissues 

 those in which oxidation is going on rapidly must 

 warm the blood which is passing through them. This 

 transmission of heat to the blood limits the rise of tem- 

 perature in such tissues and the blood imparts some of 

 the heat to less active regions of the body. It also 

 carries heat to the surfaces of the skin and the respiratory 

 tract through which it can be dissipated. 



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